// modpack server guide
Undertale server hosting & setup guide
An Undertale server needs Java 21 and about 2–3 GB of RAM for up to 8 players.
Minecraft Full Undertale map Experience (Part 1-2)
Summary from Undertale on Modrinth.
Undertale is a Fabric modpack for Minecraft 1.21 with 39 mods. We'd pick our Standard plan (4 GB) for it.
Quick facts
| Loader | Fabric |
|---|---|
| Minecraft versions | 1.21 |
| Java needed | Java 21 for Minecraft 1.21 Java version chart. |
| Mods | 39 modsCounted from the pack's file list on Modrinth. None are marked client-only. |
| RAM | About 2–2.5 GB for 1–3 playersBy player count. |
| Downloads | 168,750 on Modrinth |
| Last updated | May 23, 2025 (version 0.0.5) |
| First published | January 2, 2024 on Modrinth |
| Server support | Required: the pack has mods that must run on the server. |
| Server pack | Not published separatelyHow to build the server. |
| Modrinth tags | adventure, challenging, combat, optimization, technology |
Undertale server files
No. Undertale has no separate server pack on Modrinth, so the server is built from the .mrpack: the Fabric server for Minecraft 1.21, plus the pack's overrides folder and the server-side mods its modrinth.index.json lists. A modpack installer does this for you; our guide shows how to make a server for a modpack with no server pack.
How much RAM does an Undertale server need?
| Players online | RAM | DSN plan |
|---|---|---|
| 1–3 players | About 2–2.5 GB | Standard (4 GB) |
| 4–8 players | About 2–3 GB | Standard (4 GB) |
| 9–15 players | About 2.5–4.5 GB | Premium (8 GB)Standard (4 GB) can do if players stay close. |
Estimated from 39 mods on the server with our RAM calculator, not measured: low end if players stay together, high end if they spread out.
Set Java's heap (-Xmx) 10–15% under the RAM you have, about -Xmx3500M on 4 GB. Fabric 1.21 takes it in the java command (-Xmx), usually in the start script. RAM guide.
Undertale server requirements
| Java | Java 21 |
|---|---|
| Loader | Fabric for Minecraft 1.21 |
| CPU | Fast single-core speed (one main thread). |
| RAM | About 2–4.5 GB for 1–15 players (table above) |
| Disk | The server files, a growing world and backups |
| Network | TCP port 25565 open to players |
Recommended plan
Our pick for Undertale
Standard
4 GB RAM · planned launch price $16/month
At about 39 Fabric mods this is a light pack, so 4 GB of RAM covers it for a small group.
Ordering is closed until we relaunch. We aren't taking orders or payments while we move to new hardware. Join the launch list and we'll email you when Standard servers open.
More than 8 players, or lots exploring at once? Go up to Premium (8 GB). Not sure? Email hello@dsnservers.com.
Planned launch price in USD, billed monthly once ordering opens, with a 7-day refund window.
How to make an Undertale server on your own PC
What's specific to Undertale, step by step. The full walkthrough for any pack: how to make a modded Minecraft server.
- Install Java 21.
Check with
java -version. Java version chart. - Get the server files.
Build them from the .mrpack, as in server files above.
- Start it and accept the EULA.
Run the start script in the server files, or
java -jaron the Fabric 1.21 server launcher, then seteula=trueineula.txtif you accept Mojang's EULA (EULA step). - Set the RAM with
-Xmx, sized from the table above. - Let friends connect: forward TCP 25565, or use a tunnel or host behind CGNAT (can't connect?).
Or run Undertale on DSN
Once we relaunch (join the launch list), on DSN you'd install Undertale from the panel's Modrinth modpack installer (version 0.0.5), pick Java 21 on the Startup tab and accept the EULA in the console. No port forwarding.
How to play Undertale with friends
Each player installs Undertale version 0.0.5 with the Modrinth App or Prism Launcher, the same version as the server, and adds the server's address under Multiplayer.
Different versions of Undertale on the server and a client show up as Mismatched mod channel list or mod rejections. Why friends can't join, timeouts included.
Lag and performance on Undertale
Can't keep up! Is the server overloaded? means ticks take longer than 50 ms; more RAM won't fix that once Undertale has enough (tick lag guide). For Undertale:
- Measure first: spark's
/spark tpsand profiler work on 1.21 and name the mod or machine using the tick time. - Distances:
view-distance=8and a lowersimulation-distanceinserver.properties. - Pre-generate: Chunky for 1.21,
/chunky radius 3000then/chunky start, before people explore.
Common Undertale crashes
What a Fabric 1.21 server running Undertale hits most. For any other error, paste the log into the free crash log analyzer.
- Wrong Java version. Undertale on Minecraft 1.21 needs Java 21. Started on Java 17, it stops with
UnsupportedClassVersionError: ... class file version 65.0, this version of the Java Runtime only recognizes class file versions up to 61.0. Java version fix. - Out of memory. Undertale (39 mods on the server) needs about 2–2.5 GB for 1–3 players;
java.lang.OutOfMemoryErrormeans it ran out. Out-of-memory fix. - Client-only mod on the server. The Undertale pack file marks none of its mods client-only, so this usually means one was copied over from a player's mods folder. Check a mods list with the client-or-server-side checker. Client-only mod fix.
- After adding or updating a mod. Mods you add to Undertale by hand usually crash it with a missing dependency or a mixin conflict.
- Ticking entity or block entity. The crash report names it and gives its coordinates. Ticking entity fix.
Updates and support
Before moving Undertale past version 0.0.5, back up the world: see updating a modpack server without losing your world.
Bugs in Undertale itself go to its authors: issue tracker. Server problems on DSN: hello@dsnservers.com or Discord.
Packs like Undertale
More: all modpack guides and how to make a modded server.
Run Undertale on DSN when we relaunch.
Our pick, Standard: 4 GB, planned at $16/month.
DSN Servers isn't affiliated with Undertale or Modrinth. The pack's name, icon, summary and stats come from Modrinth's public API (checked October 4, 2026).